1. Field of the Invention
This invention relates generally to detecting and correcting drift of a semiconductor substrate from a nominal position.
2. Description of the Related Art
During processing, a robot is commonly used to transport a substrate, such as a silicon wafer, from one location to another inside a semiconductor processing machine. Generally, wafers are transported between storage cassettes and boats or wafer holders inside a processing chamber within the semiconductor processing machine. The robot includes an end effector to pick up the wafer from the cassette, transfer and place the wafer into the processing chamber and then transfer the wafer back into its storage cassette after processing is complete.
The wafer must often be placed with great accuracy. For example, there is often little tolerance for placement of wafers in the slots of a boat for a vertical furnace. See, e.g. U.S. Pat. No. 5,407,449, by Zinger.
Another example of the need for placement accuracy is illustrated in FIG. 1. A typical wafer and a susceptor for holding the wafer within a single-wafer processing chamber are depicted therein. For a given wafer, the pocket on a susceptor into which the wafer fits generally has a diameter only slightly larger than that of the wafer. There is often a very small clearance between the edge of the wafer and the edge of the susceptor pocket. It is important that the wafer be centered in the pocket and not touch the sidewalls thereof. If the wafer has contact with the sidewalls of the pocket, local temperature changes occur, resulting in temperature gradients across the wafer. This can cause non-uniformity in process results, as most semiconductor processing depends critically on temperature. Similarly, uncentered wafers can be damaged during placement in a number of different handling situations.
The wafer does not normally change position with respect to the end effector during wafer transport. Errors in final placement of the wafer, known as “drift,” are due mainly to variations in wafer position at pickup, i.e., the end effector attaches to each wafer at a slightly different location. Therefore it is necessary to correct the position of the wafer before it is placed at its destination.
Often, standalone stations are established for locating the center of a given wafer before it is picked up again by the robot, such that centered placement on the robot end effector is assured. Unfortunately, such systems require separate drop-off and pick-up operations which consume valuable processing time. It is therefore advantageous to measure and correct the wafer's position “in line” as the effector transfers the wafer from one location to another. One method of effecting this correction is by altering the drop-off point for the wafer transfer robot based on measurements of the wafer position after it is removed from the cassette.
In the prior art, there are many ways to measure the position of the wafer on the robot before the wafer is placed on the susceptor or other destination. It is desirable to avoid contact with the wafer, so optical systems are widely used. In a typical optical system, a light beam is aimed at the wafer, and sensors detect either a reflected beam or a portion of a transmitted beam as the robot moves inside the machine. Sensor data is then used to determine the wafer position.
Most methods used to correct the wafer position are based on optical through beam sensors. A typical optical sensor consists of a transmitter and a receiver. The transmitter generates an optical ray (which may or may not be within the visible spectrum) which is picked up by a receiver. If the beam is blocked by an object between the transmitter and the receiver, such as a wafer, the sensor detects the interruption and sends a signal to a computer. The computer can then extrapolate the position of the wafer based on the time and duration of interruption at the optical sensor, the speed of robot movement, and of the robot position. The actual wafer position is thus calculated and the subsequent placement operation uses this actual wafer position in order to properly place the wafer at its destination.
The accuracy of the optical measurements depends, in part, on how well the position of these optical components are known. Currently, these systems are positioned using complicated mechanical means, which are not always accurate. Moreover, typical in line wafer centering systems are rather complex and expensive, and require many sensors to be accurately positioned.
A need exists for a simple and reliable system for properly positioning wafers and other substrates during robotic transfer.